It's been a minute since I was in fluid dynamics class, but I would think that a thinner profile would be preferred on the leading edge of the plenum divider. You are trying to divide the air as efficiently as you can, and a thin profile should produce less resistance to flow and subsequent turbulence down the port behind it. Remember that an airfoil or wing has a very different job than the divider, they are producing lift and have to have the longer path on one side. When you look at the tail surfaces that provide directional stability, they are thinner in cross section and more equal length top to bottom.